It is well known that filters in a waste water treatment process carried out in e.g. a Membrane Bio Reactor (MBR) suffer from the draw backs of fouling and clogging leading to limited and even no flow through the filters. Filtering is often provided by micro- or ultra-filtration comprising membranes having very high production costs, and at the same time quite fragile filter designs. The high production costs stipulate regeneration of the membrane, which is provided by back-flushing, chemical or enzymatic cleaning.
Recent developments (See e.g. Characteristics of a Self-Forming Dynamic Membrane Coupled with a Bioreactor for Municipal Waste water Treatment, Environ. Sci. Technol. 2002, 36, 5245-5251) suggest that the layer of biomass on a coarse mesh improves a solid-liquid separation. In the suggested method, the thickness of the biomass layer grows continuously during filtration. In order to control growth, bottom aeration is used to continuously lift some solids from the surface and the biomass layer and to keep the thickness more or less constant. Thickness of the fouling layer on the membrane depends on many factors: coarse bubble aeration rate, filter module geometry, sludge properties etc. so it is very hard to choose optimum conditions; in fact, the cake, or in general the biomass layer, forms more or less spontaneously, so its rejection capabilities are to a large extent undefined. Also, membrane coarse bubble aeration is one of the largest contributors to operating costs of an MBR, and membranes need chemical cleaning, which is both expensive and complicated.
An example of such a waste water treatment system is disclosed in JP 2004167393 which disclose a system in which a filter surface is exposed to gas bubbles by use of a diffuser arranged below a filter element to keep the filter element clean.
Hence, an improved means of achieving biomass retention in a waste water treatment process would be advantageous, and in particular a more efficient and/or reliable filtration operation mode would be advantageous.
It is also a known fact that biomass characteristics in typical MBR systems, and in particular the average particle size and particle size distribution, limit especially the filterability of biomass. Since these properties are uniform throughout the traditionally-built systems, they are never optimal for the different unit operations. Hence an additional aim of the present invention is to improve and optimize the waste water treatment process by adjusting the biomass flocculation state to the unit operations that the biomass is subjected to.
It is a further object of the present invention to provide an alternative to the prior art. In particular, it may be seen as an object of the present invention to provide devices, systems and methods that solve the above mentioned problems of the prior art with a new method for achieving membrane-quality filtration with compressed biomass, and a method to adjust biomass particle size distribution so that it is optimal for each unit operation in a waste water treatment process.